1. Why SF6 gas has excellent heat transfer properties?
Ans. SF6 has excellent heat transfer properties because it has high molecular weight and low gaseous viscosity. Its heat transferrability at atmospheric pressure is 2 to 2.5 times that of air.
2. Why chances of arc interruption, in subsequent current zero, decrease in case of air blast circuit breaker but increase in oil circuit breaker?
Ans. After the first current zero air pressure is reduced in ABCD due to which chances of arc interruption decrease. But in case of oil C.B. gap length increases and dielectric strength builds up after first current zero so chances of arc interruption increase.
3. What is the current chopping? How it can be minimised?
Ans. Current Chopping in circuit breaker is defined as a phenomena in which current is forcibly interrupted before the natural current zero. Current Chopping is mainly observed in Vacuum Circuit Breaker and Air Blast Circuit Breaker.This results in severe overvoltages across. Various CBs have different provisions to avoid current chopping.
Current chopping in minimised using resistance switching. Inclusion of resistance across the CB reduces the natural frequency and therefore RRRVmax and makes the circuit critically damped due to which transients vanish quickly.
4. For rural electrification in a country like India with a complex network, which C.B. should be preferred?
Ans. Vacuum, C.B. because of the short gap and excellent recovery characteristics of vacuum breakers, they can be used where the switching frequency is high and required to be reliable. For low fault interrupting capacities, the cost is low as compared to other interrupting devices.
Current interruption occurs at the first current zero after contact separation with no re-striking, making it exceptionally good for capacitor and cable switching and long line dropping.
Very high power frequency and impulse withstand voltages with small contact spacing, allowing ease of actuation and timing.
5. what is the difference between lightning arrester and rod gap?
Ans. The main difference between these is -
Rod gap can’t interrupt power frequency follow currents due to which every operation of rod gap results in a line to ground fault. But expulsion type of lighting arrester can interrupt power frequency follow current.
6. What is the basic requirement of a surge diverter?
Ans.A surge diverter is a piece of equipment that diverts excess voltages (caused by spikes in the electrical supply) to earth, thus protecting sensitive electrical equipment. The basic requirement of a surge diverter are :
- It should breakdown as quickly as possible after the abnormal high-frequency voltage arrives.
- It should not pass any current at normal power frequency voltage.
- It should not protect the equipment for which it is used but should discharger surge current without damaging itself.
7. What is count poise?
Ans. A counterpoise is a galvanized steel wire run in parallel or radial or a combination of the two, under the ground, with respect to the overhead line. It is the surge impedance of the counterpoises which is important initially and once the surge has travelled over the counterpoise it is the leakage resistance of the counterpoise that is important. Leakage resistance of counterpoise should be less than surge impedance.
8. Why ground wire is used on the top of the tower?
Ans. It protect power conductors form the direct lightning stroke. Whenever a direct lightning stroke falls on the tower, the ground wire on both sides of the tower provides a parallel path for the stroke thereby the effective impedance is reduced and the tower top potential is relatively less.
9. What is dynamic stability?
Ans. The ability of a power system to maintain stability under continuous small disturbances is investigated under the name of Dynamic Stability (also known as small-signal stability). These small disturbances occur due random fluctuations in loads and generation levels. In an interconnected power system, these random variations can lead catastrophic failure as this may force the rotor angle to increase steadily.
10. Why load flow studies are carried out?
Ans. Load flow studies help us in planning the new network and expanding the existing one.
11. What are the uses of Stringing charts ?
Ans. Sag and tension. It is a plot between sag and tension versus temperature.For use in the field work of stringing the conductors, temperature-sag and temperature tension charts are plotted for the given conductor and loading conditions. Such curves are called stringing charts
12. For the lower cost of electricity generation what should be load factor and diversity factor?
Ans. Both load factor and diversity factor should be high.A higher value of load factor means, less maximum demand. Less maximum demand can be catered by a low capacity power plant.A high diversity factor has the effect of reducing the maximum demand.
13. Why charging current in transmission line increases due to corona effect?
Ans. Due to corona effective radius of conductor increases .
and hence capacitance increases.
14. What is penstock?
Ans. It is a conduit system for taking water from forebay to powerhouse. It is made up of steel for high and medium head plants but it may be of concrete structure for low head plants.
15. What is the function of economizer in thermal power plants?
Ans.A common application of economizers in steam power plants is to capture the waste heat from boiler stack gases (flue gas) and transfer it to the boiler feedwater. This raises the temperature of the boiler feedwater, lowering the needed energy input,
Economizer heat feed water by extracting heat from flue gases. Heat which is carried by flue gases would be otherwise wasted in the atmosphere. This heated water is then fed to the boiler. Thus it increases the efficiency of thermal power plant.
16. What is co-generation?
Ans. Co-generation is the simultaneous production of electricity and heat, both of which used.
Co-generation first involves producing power from a specific fuel source, such as natural gas, biomass, coal ,or oil . During fuel combustion, co-generation captures the excess heat which would have otherwise been wasted. The captured heat can be used to boil water, create steam, heat buildings, etc.
17. What is the Grid? How many Regional Grids are in India?
Ans. An electrical grid is an interconnected network for delivering electricity from suppliers to consumers. It consists of generating stations, transmission lines and distribution lines.
There are five regional grids in India:
- Northern
- Southern
- Eastern
- North-Eastern
- Western
18. How much land is used for Solar Energy Production?
Ans. 1 MW solar PV power plant requires nearly 4.5-5 acres for crystalline technology and around 6.5-7.5 acres for thin film technology.
19. why skin effect does not exist in HVDC?
Ans. Factors affecting the skin effects is -
since skin effect is inversely proportional to the skin depth.
skin depth is given by -
, 
⟹ in case of HVDC, f = 0 , so skin depth is infinite which mean zero skin effect.
∴ effect of skin depth is totally eliminated.
20 What is the purpose of use of steel in ACSR conductor?
Ans. Steel possesses higher mechanical strength than aluminium which allows for increased mechanical tension to be applied to the conductor.
21. Explain the mechanism of SCADA?
Ans. SCADA( supervisory control and data acquisition, a computer system ) software receives the input from Programmable Logical Controllers (PLC’s) or Remote Terminal Units (RTU’s) which in turn receive their information from the sensors or input values which we have given manually. SCADA is used for gathering and analyzing real time data.
Use of SCADA in Power system:
- Continuous monitoring of speed and frequency
- Load scheduling
- Power factor correction
22. Difference between Newton Rapson and Gauss seidel ?
Ans. Difference between Newton Rapson and Gauss seidel-
23. How do we conserve electricity in Home?
Ans. We can conserve electricity in Home by the following means:
- Replacing existing bulbs with energy efficient LED’s.
- Turning-off lights when not in use.
- Replacing older appliances with energy-saving models based on star ratings.
- Use renewable energy sources.
24. What you will do if generations are higher than load demand if you work in a distribution company?
Ans. If any load restrictions were imposed, they should be lifted and consumers should be encouraged by giving more incentives. Supply of power from surplus substation to the deficit one can take place by interlinking feeders.
25. What is the condition for active power flow in the transmission line?
Ans. For active power to flow, there must be an angular difference between the 2 bus voltages among which power is to be transferred.
current in the transmission line is
for active power flow in the transmission line current must be flow.
26. What is the condition for reactive power flow through the transmission line?
Ans. For reactive power to flow between two buses, the magnitude of the voltages of the two buses must be different.
27. What are distance protection scheme?
Ans.Distance protection relay is the name given to the protection, whose action depends on the distance of the feeding point to the fault. The time of operation of such protection is a function of the ratio of voltage and current, i.e., impedance. This impedance between the relay and the fault depends on the electrical distance between them . The key advantage is fault coverage of the protected circuit is virtually independent of source impedance variations.
28. What is Automatic voltage Regulator & voltage stabilizer?
Ans. AVR is the controller to regulate voltage deviations within 5% changes voltage (or) the steady-state changes. Whenever the voltage is decreases at the generator terminals the AVR increases the alternator excitation so that the voltage at terminal can increases.
A voltage stabilizer is an electrical appliance that feeds constant voltage to a load during over and under voltage conditions.
29. What are Conventional and Non-conventional energy sources?
Ans. Conventional energy means the energy source that is fixed in nature oil, gas and coal. These are also called as non-renewable energy sources. Non-conventional sources are those energy sources which are renewable and ecologically safe such as solar energy, wind energy etc.
30. What is floating neutral? How do we overcome this problem?
Ans. When ever the neutral is not connected to ground due to third harmonics neutral experiences a variable voltage at different time. Floating neutral condition in an unbalanced network can cause overvoltages.
To detect it, the neutral loss detection relay is installed at the in-comer of distribution boards to detect the floating neutral condition.
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